1. Field of the Invention
This invention relates to a structure of a panel. As used herein, the term "panel" means a flat or plate material having a smoothly curved surface throughout the material. For example, the panel may include an outside plate of a vehicle body or body such as a roof panel, a hood panel, a door outer panel, a quarter panel or a fender panel, and an inside plate of the body such as a floor pan, a dash panel, an upper back panel or a lower back panel. Further, the panel may include a panel of an instrument such as a fuel tank, and a non-metal panel such as a front windshield glass, a back window glass or a door glass of the vehicle. Moreover, the panel may include a roof and a window glass of a railway vehicle, and a window glass of a building.
2. Description of the Prior Art
For example, in a roof of a vehicle having a panel, the panel 10 is generally formed as including a curved surface provided with an intermediate portion 11 and a peripheral portion 12, the intermediate portion 11 having a curvature less than the curvature of the peripheral portion 12 as shown by the dotted line of FIG. 1. This is mainly based on demand for design.
As a special panel, there is provided a panel which is made of plastic so as to have a thickened intermediate portion and a thinned peripheral portion, a thickness of the panel gradually becoming thinner from the intermediate portion toward the peripheral portion (See Japanese Utility Model Public Disclosure (KOKAI) No. 61-177984).
In general, the thickness of the panel is determined depending upon the degree of deflection of the panel, i.e., panel stiffness at a time when applying , a predetermined load. However, in conventional panels, the thickness of the panel is determined on the basis of the intermediate portion having the smallest panel stiffness since the intermediate portion and the peripheral portion have each different panel stiffness due to the different curvature on the panel surface. As a result, the peripheral portion is provided with an excess thickness so that the panel becomes heavy excessively.
The panel stiffness of the intermediate portion may be increased by using the panel having the thickened intermediate portion and the thinned peripheral portion, the thickness thereof becoming thinner from the intermediate portion toward the peripheral portion as disclosed in the above-mentioned Japanese Utility Model Public Disclosure. However, the weight of the panel is increased correspondingly to the increased thickness of the intermediate portion. Further, when the panel is made of steel plate, glass or the like which is difficult to use injection molding or extrusion molding, the manufacture of the panel is complicated.